1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to an apparatus and method for uplink channel sounding.
2. Description of the Related Art
Multiple-Input Multiple-Output (MIMO) is under intense study as one of the promising technologies for high-speed data transmission.
MIMO techniques are categorized into open-loop MIMO and closed-loop MIMO. In open-loop MIMO techniques, including Space-Time Coding (STC) and Vertical-Bell Labs Layered Space-Time (V-BLAST), a transmitter transmits data without knowledge of channel information. In closed-loop MIMO techniques, such as Singular Value Decomposition (SVD) and Spatial Division Multiple Access (SDMA), the transmitter acquires channel information and transmits data based on the channel information.
How the transmitter acquires the channel information depends on the duplexing mode. In Frequency Division Duplexing (FDD), a receiver measures the channel and feeds back the measured channel information to the transmitter. In Time Division Duplexing (TDD), the receiver transmits channel sounding signals to the transmitter and the transmitter measures the channels using the channel sounding signals and applies the uplink channel information to downlink channels. This is possible by exploiting the channel reciprocity of TDD between downlink and uplink transmissions.
A scenario where a Base Station (BS) uses a plurality of antennas and a Mobile Station (MS) uses a single antenna is a typical one considered for real implementation of a MIMO system because of the limit of the physical distance between antennas and the complexity and cost of the MS. Nonetheless, this system is called a MIMO system rather than a Multiple-Input Single-Output (MISO) system in that the BS can communicate with a plurality of users, i.e. a plurality of antennas when SDMA, for example, is used. In this scenario, the MS transmits and receives signals in the same manner as done in a conventional single-antenna system and the BS performs transmission and reception in MIMO.
FIG. 1 illustrates an uplink channel sounding scheme for a single-antenna MS.
Referring to FIG. 1, each MS 101 transmits a channel sounding signal distinguishable from those of other MSs to a BS 103. BS 103 measures the uplink channel of MS 101 using the channel sounding signal and uses the measured uplink channel information as downlink channel information in downlink data transmission.
Another scenario for a real MIMO system is that the MS uses a single Transmit (Tx) antenna and two Receive (Rx) antennas. The use of the two Rx antennas offers the benefits that diversity gain increases cell coverage and that as the BS can transmit two streams to the MS, the downlink data rate for the MS increases.
In general, the MS transmits at a far lower power level than the transmit power of the BS due to the lifetime of its battery. Thus, the MS can be designed to have two Rx antennas and one Tx antenna. For example, in the illustrated case of FIG. 2, the MS can use one physical antenna 207 (second antenna) as an Rx antenna and another one 205 (first antenna) for the dual purpose of transmission and reception. In this case, a Tx-Rx switch 203 is provided between the Tx-Rx antenna 205 and a baseband Modulator-Demodulator (MODEM) 201, for switching between transmission and reception.
FIG. 3 illustrates an uplink channel sounding scheme for the above-described multi-antenna MS.
Referring to FIG. 3, an MS 301 transmits channel sounding signals to a BS 303 through the Tx-Rx antenna 205. BS 303 measures uplink channels using the channel sounding signals. However, the measured uplink channel information does not suffice for use as downlink channel information because it is confined to uplink channels between Tx-Rx antenna 205 and BS 303 with no regard to uplink channels between Rx antenna 207 and BS 303.
In the above scenario with one Tx antenna and two Rx antennas in an MS, given Nt antennas in a BS, 2Nt downlink channels exist for Nt uplink channels. Therefore, the BS does not get full knowledge of the downlink channels using channel sounding signals received from the MS.